1. Field of the Invention
The present invention relates to an imaging lens and, particularly, to an imaging system of three-lens structure, which is capable of reducing the size and weight. The imaging system is used for an image-taking device that uses an image sensor element such as a CCD, CMOS, etc.
2. Description of the Related Art
Recently, there has been an increasing demand for cameras that utilize an image sensor element such as a CCD, CMOS, or the like, which are mounted on a portable computer, a television phone, a portable phone, and a digital camera, for example. It is demanded that a camera such as this is small and light because the camera is required to be mounted on a limited installation space.
Therefore, it is also necessary for the imaging lens system used for such cameras to be small and light, as is the camera. Conventionally, a single lens structure lens system using a single lens and a two-lens structure system that uses two lenses are used as such imaging lens systems.
These imaging lens systems are very advantageous for reducing the size and weight of the lens system. However, these imaging lens systems cannot handle higher definition and higher resolution that is demanded of the imaging lens system in recent years.
Therefore, conventionally, a three-lens lens system using three lenses is utilized to handle higher definition and higher resolution.
As the lens system with three-lens structure aiming for higher definition and higher resolution, there have been known the lens systems disclosed in Patent Literatures 1–4.
[Patent Literature 1] Japanese Patent Unexamined Publication 2005-173298
[Patent Literature 2] Japanese Patent Unexamined Publication 2005-91513
[Patent Literature 3] Japanese Patent Unexamined Publication 2003-322792
[Patent Literature 4] Japanese Patent Unexamined Publication 2004-163786
However, in the lens system in Patent Literature 1, a third lens is a negative lens whose convex surface faces the object side. Therefore, it is difficult to balance between aberration correction and size reduction of the lens system. In addition, the power distribution of first to third lenses and the center radius curvature of both surfaces of the first lens in the lens system in Patent Literature 1 cause the back focus distance to become too long. As a result, the entire length of the lens system becomes too long.
In addition, as in the lens system in Patent Literature 1, the power distribution of the first to third lenses in the lens system in Patent Literature 2 causes the back focus distance to become too long. As a result, the entire length of the lens system becomes too long.
Further, in the lens system in Patent Literature 3, the third lens is a negative lens whose convex surface faces the object side. Therefore, as in the lens system in Patent Literature 1, it is difficult to balance between aberration correction and reducing the size of the lens system. In addition, the power distribution of the first to third lenses in the lens system in Patent Literature 3 causes the overall length of the lens system to become too long. In addition, in the lens system in Patent Literature 3, the back focus distance is short and the third lens has a meniscus shape. Therefore, the last surface in the end of the optical surface protrudes significantly towards the image surface side, thereby causing a problem when various filters and the like are inserted.
Still further, the power distribution of the first, second, and third lenses in the lens system in Patent Literature 4 causes the entire length of the lens system to become too long.
Therefore, the conventional lens systems are still insufficient for maintaining an excellent optical performance, such as high-definition and high-resolution, while actualizing a further reduction in the size and weight (shortening of the entire length) of the lens system itself.
The present invention has been designed in view of the aforementioned problems. The object of the present invention, therefore, is to provide an imaging lens that can maintain an excellent optical performance, while actualizing a further reduction in the size and weight.